Make DeadArgElim keep liveness of the return value as a whole in addition to

Make DeadArgElim keep liveness of the return value as a whole in addition to
only the liveness of partial return values (for functions returning a struct).
This is more explicit to prevent unwanted changes in the return value.

In particular, deadargelim now canonicalizes a function returning {i32} to
returning i32 and {} to void, if the struct returned is not used in its
entirety, but only the single element is used.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@53606 91177308-0d34-0410-b5e6-96231b3b80d8